Transparent styrol-butadiene block copolymer mixtures

ABSTRACT

Mixtures comprising linear block copolymers made from vinylaromatic monomers and from dienes of the structure (I) S 1 —B 1 —S 2  and (II) S 3 —B 2 —S 4 , where S 1  is a block made from vinylaromatic monomers with number-average molar mass M n  in the range from 40,000 to 100,000 g/mol, each of S 2 , S 3  and S 4  is a block made from vinylaromatic monomers with number-average molar mass M n  in the range from 5,000 to 20,000 g/mol, each of B 1  and B 2  is a block made from dienes or a copolymer block made from dienes and from vinylaromatic monomers with number-average molar mass M n  in the range from 15,000 to 40,000 g/mol, and the ratio of the block copolymers (I)/(II) is in the range from 0.6 to 2.

The invention relates to mixtures comprising linear block copolymersmade from vinylaromatic monomers and from dienes of the structure (I)S₁—B₁—S₂ and (II) S₃—B₂—S₄, where S₁ is a block made from vinylaromaticmonomers with number-average molar mass Mn in the range from 40,000 to100,000 g/mol, each of S₂, S₃ and S₄ is a block made from vinylaromaticmonomers with number-average molar mass M_(n) in the range from 5,000 to20,000 g/mol, each of B₁ and B₂ is a block made from dienes or acopolymer block made from dienes and from vinylaromatic monomers withnumber-average molar mass M_(n) in the range from 15,000 to 40,000g/mol, and the ratio of the block copolymers (I)/(II) is in the rangefrom 0.6 to 2. The invention further relates to processes for preparingthe mixtures, and to their partial or complete hydrogenation.

Styrene-butadiene block copolymer and mixtures with polystyrene areknown, with a variety of structures. The block copolymers may be linearor have star-type branching, and may have blocks of identical ordifferent molar masses, the result being symmetrical or asymmetricalstructures. The butadiene-containing blocks may also contain styrene.There may be sharp or tapered transitions between the individual blocks.An overview of styrene-butadiene block copolymers is found by way ofexample in Kunststoff Handbuch, Vol. 4 Polystyrol, Carl Hanser-VerlagMunich, Vienna, 1996, Chapter 3.3.4, pages 161-164.

DE-A 29 40 861 discloses mixtures of linear S-B-S three-block copolymerswith varying compositions and molar masses. The mixture is obtained bysequential anionic polymerization using two joint feeds of initiator andstyrene. The ratio of the amount of initiator in the first stage to thatin the second stage is in the range from 1:2 to 1:7, implying that thereis a marked preponderance of the block copolymer having the relativelyshort styrene block. The transition between the first styrene block andthe butadiene-containing block is sharp, but the transition from thebutadiene-containing block to the second styrene block is gradual.

U.S. Pat. No. 5,227,419 describes mixtures of block copolymers whosebutadiene-containing blocks have a styrene gradient. Again, a mixturecomprises a subordinate amount of the block copolymer having therelatively high styrene block content.

However, in mixtures with polystyrene, the block copolymers describedlead to drastically reduced stiffness compared with that of polystyrene,while toughness is comparable. There is also a marked lowering of heatresistance.

Styrene-butadiene block copolymers and styrene-isoprene block copolymersmay be hydrogenated to give polymers with different properties, forexample with improved aging resistance or improved weatheringresistance. Depending on the hydrogenation conditions, the olefinicdouble bonds may be hydrogenated selectively here (U.S. Pat. No.4,882,384), or else both the olefinic and the aromatic double bonds maybe hydrogenated (U.S. Pat. No. 3,333,024, U.S. Pat. No. 3,431,323, U.S.Pat. No. 3,598,886).

WO 94/21694 describes, by way of example, the hydrogenation ofpolystyrene or of styrene-butadiene block copolymers, or ofstyrene-isoprene block copolymers, on supported metal catalysts. Underthe conditions of the reaction, it is not only the diene block which ishydrogenated, but also the phenyl groups of the polystyrene block. Apolycyclohexylethylene (PCHE) block is thus produced from thepolystyrene block.

WO 96/34896 describes an improved hydrogenation catalyst for thering-hydrogenation of styrene polymers. As starting material for thering-hydrogenation, that specification uses not only polystyrene, butalso two- and three-block polymers composed of styrene and butadiene orof styrene and isoprene. The hydrogenation of styrene-butadiene blockcopolymers or styrene-isoprene block copolymers having 3 and,respectively, 5 blocks (WO 00/32646, WO 00/56783, WO 01/12681) isdescribed, as is also the hydrogenation of styrene-butadiene star blockpolymers (WO 01/23437).

EP-A 505 110 discloses hydrogenated mixtures composed ofstyrene-butadiene block copolymers and polystyrene for optical storagemedia.

It is an object of the present invention to provide transparent mixturesof styrene-butadiene block copolymers with polystyrene which do not havethe abovementioned disadvantages and which in particular have higherstiffness and higher heat resistance, with comparable toughness. Afurther object of the present invention was to provide ring-hydrogenatedblock copolymers which, especially in a blend with ring-hydrogenatedpolystyrene, has not only a very good toughness/stiffness ratio andexcellent transparency, but also high heat resistance. They should alsohave good compatibility with hydrogenated polystyrene, in order toprovide homogeneous mixtures with excellent transparency.

We have found that this object is achieved by means of theabovementioned mixtures.

The ratio of the block copolymers (I)/(II) is in the range from 0.6 to2, preferably in the range from 0.7 to 1.5, particularly preferably inthe range from 0.9 to 1.3.

Examples of vinylaromatic monomers which may be used are styrene,alpha-methylstyrene, ring-alkylated styrenes, such as p-methylstyrene ortert-butylstyrene, or 1,1-diphenylethylene, or a mixture of these.

Preferred dienes are butadiene, isoprene, 2,3-dimethylbutadiene,1,3-pentadiene, 1,3-hexadiene, or piperylene, or a mixture of these;butadiene and isoprene are particularly preferred.

Particularly preferred block copolymers are formed from the monomersstyrene and butadiene.

The blocks B₁ and B₂ may be composed exclusively of dienes or of dienesand vinylaromatic monomers. The vinylaromatic monomer/diene ratio forthe blocks B₁ and B₂ is generally in the range from 0 to 1, and thevinylaromatic monomer/diene ratio in the blocks B₁ and B₂ may beidentical or different. The blocks B₁ and B₂ are preferablyhomopolydiene blocks, in particular homopolybutadiene blocks, orcopolymer blocks with a vinylaromatic monomer/diene ratio in the rangefrom 0.3 to 0.7. The copolymer blocks particularly preferably haverandom distribution of the diene monomers and vinylaromatic monomers.

The transitions between the individual blocks are sharp, i.e. thecomposition changes suddenly. The number-average molar mass M_(n) of Siis in the range from 40,000 to 100,000 g/mol, preferably in the rangefrom 45,000 to 70,000 g/mol, particularly preferably in the range from50,000 to 60,000 g/mol. Each of the number-average molar masses M_(n) ofS₂, S₃, and S₄ is, independently of the others, in the range from 5,000to 20,000 g/mol, preferably in the range from 8,000 to 17,000 g/mol,particularly preferably in the range from 11,000 to 14,000 g/mol. Eachof the blocks B₁ and B₂ made from dienes, or the copolymer blocks madefrom dienes and from vinylaromatic monomers, independently of theothers, has a number-average molar mass M_(n) in the range from 15,000to 40,000 g/mol, preferably in the range from 18,000 to 30,000 g/mol,particularly preferably in the range from 20,000 to 25,000 g/mol.

The block copolymers (II) preferably have a symmetrical structure, i.e.the blocks S₃ and S₄ have the same number-average molar mass M_(n). Incontrast, the block copolymers (I) are markedly asymmetrical, the ratioof the number-average molar masses of S₁ and S₂ being at least 2,preferably in the range from 5 to 8.

The mixtures of the invention may be prepared by preparing each of theblock copolymers (I) and (II) by sequential anionic polymerization ofvinylaromatic monomers and dienes with organo-alkali-metal initiators insuccession or in different reactors, and then blending these with aratio (I)/(II) in the range from 0.6 to 2.

The anionic polymerization initiator used may be any of the conventionalmono-, bi- or multifunctional alkyl, aryl, or aralkyl compounds of analkali metal. It is advantageous to use organolithium compounds, such asethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-,diphenylhexyl-, hexamethyldi-, butadienyl-, isoprenyl- orpolystyryllithium, or 1,4-dilithiobutane, 1,4-dilithio-2-butene, or1,4-dilithiobenzene. The amount of polymerization initiator neededdepends on the desired molecular weight. It is generally in the rangefrom 0.001 to 5 mol %, based on the total amount of monomer.

The polymerization may be undertaken in the presence of a solvent.Suitable solvents are the conventional aliphatic, cycloaliphatic, oraromatic hydrocarbons having from 4 to 12 carbon atoms and used foranionic polymerization, such as pentane, hexane, heptane, cyclohexane,methylcyclohexane, isooctane, benzene, alkylbenzenes, such as toluene,xylene, ethylbenzene, or decalin, or a suitable mixture. Preference isgiven to the use of cyclohexane and methylcyclohexane.

The polymerization may also be carried out in the absence of solvent andin the presence of organylmetal compounds which act as retarders for thepolymerization rate, for example alkylmagnesium, alkylaluminum, oralkylzinc compounds.

Once the polymerization has ended, the living polymer chains may becapped using a chain terminator. Suitable chain terminators areprotonating substances or Lewis acids, for example water, alcohols,aliphatic or aromatic carboxylic acids, or else inorganic acids, such ascarbonic acid or boric acid.

The blending of the block copolymers may take place at any desiredjuncture once the polymerization has ended, e.g. prior to or aftertermination, devolatilization, or other work-up steps. Thechronologically or spatially separate preparation of the blockcopolymers (I) and (II) has the advantage that the number-average molarmasses M_(n) of the individual blocks S and B can be selected freely.

An alternative process permits the production of the block copolymers(I) and (II) by sequential anionic polymerization of vinylaromaticmonomers and of dienes with organo-alkaline-metal initiatorssimultaneously in a single reactor, using two joint feeds of initiatorand vinylaromatic monomers, the ratio of the amount of initiator I₁ inthe first feed to the amount of initiator I₂ in the second feed being inthe range from 0.6 to 2. After each feed, polymerization is carried outto complete conversion of the monomers. Each of the mixtures obtained bythis process has the same number-average molar mass M_(n) for the blocksS₂ and S₄ and the same number-average molar mass M_(n) for the blocks B₁and B₂. Table 1 summarizes the feed sequence and the polymer speciesformed: TABLE 1 Feed sequence with two initiator feeds StageMonomers/Initiator Species formed 1 Initiator (I₁) and vinylaromaticmonomer 2 Initiator (I₂) and vinylaromatic S₁-I₁ monomer S₃-I₂ 3 Dieneor diene and vinylaromatic S₁-B₁-I₁ monomer S₃-B₂-I₂ 4 Vinylaromaticmonomer S₁-B₁-S₂-I₁ S₃-B₂-S₄-I₂ 5 Terminator, e.g. isopropanol S₁-B₁-S₂S₃-B₂-S₄

If a mixture made from diene/vinylaromatic monomer is used in stage 3,random distribution of the vinylaromatic monomers and dienes in theblocks B₁ and B₂ may be achieved by adding Lewis bases, such astetrahydrofuran, or potassium salts, such as potassiumtetrahydrolinaloolate.

The mixtures of the invention made from the linear block copolymers (I)and (II) can be used for blending the thermoplastic polymers over a widerange. Preferred mixtures comprise from 5 to 95 percent by weight of thelinear block copolymers (I) and (II) and from 95 to 5 percent by weightof standard polystyrene (GPPS) or impact-modified polystyrene (HIPS).Mixtures of this type may be prepared by compounding during thedevolatilization of the block copolymers, for example by addingpolystyrene in the form of “Coldfeed” into a vented extruder. Jointwork-up gives homogeneous ternary mixtures which are also capable ofdirect use by processors on non-mixing injection molding machinery. Asan alternative, however, mixtures of pellets may also be processeddirectly in kneaders, extruders, or injection molding machinery whichprovides mixing, to give ternary mixtures.

The mixtures have high toughness together with high stiffness. One wayin which this is apparent is in higher tensile strain at break thanconventional mixtures of styrene-butadiene block copolymers withpolystyrene, while modulus of elasticity is identical. They aretherefore especially suitable for injection molding, and can be used fordesigns which save material, since they have a good toughness/stiffnessratio. The mixtures may be processed to give tough moldings, for exampletransparent clothes hangers which have very good dimensional stabilityeven at relatively high temperatures.

Preference is also given to applications in extrusion, for example forproducing films for thermoforming, which may then be thermoformed togive cups, lids, lunch boxes, or other containers. Here, the hightoughness/stiffness ratio permits the use of thinner films withretention of strength, giving significant savings in materials.

The inventive block copolymer mixtures may be modified via partial orcomplete hydrogenation. The degree of hydrogenation of the olefinicdouble bonds is generally 97% or higher, and the degree of hydrogenationof the vinylaromatic monomers is preferably at least 90%, particularlypreferably at least 95%, in particular 98%.

The proportion of the copolymerized diene units present in the 1,2-vinylform may be controlled via the addition of substances with donorproperties, for example ethers or amines.

The preferred compounds used for this purpose are tetrahydrofuran,dimethoxyethane, or 2-alkylfurfuryl ethers, in amounts of from 0.1 to 1%by volume, in particular from 0.25 to 0.5% by volume, based on thehydrocarbon used as solvent, e.g. cyclohexane.

Following the preparation of the block copolymer, the unsaturated bondsof the diene units, and also of the vinylaromatic units, of the blockcopolymer are hydrogenated, using a hydrogenation catalyst. It ispreferable to use supported hydrogenation catalysts. Examples ofsuitable support materials which may be used are inorganic substrates,such as barium sulfate, silicates, carbon, or aluminum oxides. By way ofexample, suitable hydrogenation metals are nickel, cobalt, rhodium,ruthenium, palladium, platinum, or other metals of group 8.

The hydrogenation preferably takes place in a saturated hydrocarbon assolvent in which the block copolymer is soluble. It is preferable to usecycloaliphatic hydrocarbons, in particular cyclohexane. It is advisablefor the solvent used to be the same as that used during thepolymerization, so that the hydrogenation can take place in a step whichfollows the polymerization. The hydrogenation may take place batchwiseor continuously, preference being given to continuous hydrogenation on afixed-bed catalyst.

The hydrogenation generally takes place at temperatures in the rangefrom 40 to 250° C., particularly preferably in the range from 120 to180° C. The hydrogenation may be carried out at from atmosphericpressure to 350 bar, preferably in the range from 100 to 250 bar.

EXAMPLES

The butadiene content of the block copolymers was determined by means ofIR spectroscopy. The number-average molecular weights Mn and thepolydispersities (PDIs) were determined using GPC measurement, withcalibration based on polystyrene standards. Glass transitiontemperatures were measured by means of DSC in the range of −100 to 230°C. If two glass transition temperatures are stated, these are those ofthe soft and hard phase, respectively. If only one glass transitiontemperature is stated, this is that of the hard phase.

The degrees of hydrogenation stated are based on the proportion ofhydrogenation of aromatic double bonds, determined via GPC, by comparingUV intensity prior to and after hydrogenation.

Modulus of elasticity, tensile strength, and tensile strain at breakwere determined together to ISO 527, and Charpy notched impact strengthto ISO 179-1/leA(F). The Vicat B softening point was determined to ISO306 (1994).

The transmittance measurements were made in the range from 400-700 nm onpressed plaques of thickness 1 mm.

Examples 1 to 6

The block copolymer mixtures were prepared using the information intable 2 by sequential anionic polymerization with two joint feeds ofstyrene and initiator (sec-butyllithium BuLi) in stage 1 or 2 at asolids content of about 30% by weight in cyclohexane at from 50 to 80°C. Once the polymerization had been completed, isopropanol was used fortermination and the mixture was acidified using CO₂/water. In example 6,stage 3 was carried out in the presence of potassium tert-amylalcoholate (PTA), in order to obtain a random S/B copolymer block. TABLE2 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Stage 1 BuLi1 1.226 1.381 1.3811.381 1.381 1.381 [mol] Styrene 61.23 68.98 72.01 75.06 65.95 68.98 [kg]2 BuLi2 1.631 1.381 1.381 1.381 1.381 1.381 [mol] Styrene 34.66 33.5134.98 36.47 32.04 33.51 [kg] 3 Butadiene 69.45 63.99 58.03 52.00 69.9743.27 [kg] Styrene 0 0 0 0 0 20.72 [kg] 4 Styrene 34.66 33.51 34.9836.47 32.04 33.51 [kg] Properties Butadiene 34.7 32 29 26 35 21.6 [% byweight] Structure 0.75 1 1 1 1 1 (I)/(II) M_(n)(S₁) 62073 62073 6479467544 59346 62073 M_(n)(S₂) 12131 12131 12663 13200 11598 12131 M_(n)(B₁= B₂) 24312 23165 21007 18825 25328 23165 M_(n)(S₃ = S₄) 12131 1213112663 13200 11598 12131

Examples 7 to 12

Each of the block copolymer mixtures of examples 1-6 was mixed in avented extruder with 10, 17.1, 24.4, 31.3 and 38.5 percent by weight,based on the entire mixture, of a standard polystyrene with a meltvolume ratio MVR 200/5 of 3. (grade 158 K from BASF AG). The results aregiven in table 3. TABLE 3 Modulus Tensile Tensile Mixture of stress onstrain at from Polystyrene Vicat B elasticity FR FR Yellowness Ex. Ex.[% by wt.] [° C.] Shore D [N/mm²] [N/mm²] [%] Transm. % index  7a 1 10.053.5 66.9 1646 14.5 92.8 86.4 6.5  7b 1 17.1 61.3 69.2 1745 15.2 54.879.9 14.7  7c 1 24.4 66.6 71.6 1832 16.8 27.2 68.9 25.6  7d 1 31.3 68.273.7 1920 21.2 19.1 62.9 30.0  7e 1 38.5 72.8 74.9 2044 25.1 14.2 57.333.7  8a 2 10.0 58.8 68.5 1763 13.9 39.4 85.7 6.3  8b 2 17.1 64.2 71.41849 16.0 22.5 82.1 11.3  8c 2 24.4 68.0 73.4 1918 21.1 19.5 76.8 16.9 8d 2 31.3 71.4 75.3 2025 23.8 17.3 67.1 26.9  8e 2 38.5 75.0 76.7 213425.9 13.9 63.2 29.8  9a 3 10.0 66.1 72.7 1829 19.7 28.0 81.6 10.4  9b 317.1 69.9 74.3 1917 21.2 25.0 81.4 11.4  9c 3 24.4 73.7 75.8 2008 23.021.0 79.3 14.2  9d 3 31.3 76.7 77.7 2112 25.0 16.4 75.2 19.2  9e 3 38.579.2 78.8 2209 27.3 12.6 72.1 22.7 10a 4 10.0 68.7 75.1 1881 21.3 23.986.7 5.0 10b 4 17.1 73.0 76.3 1983 23.0 21.1 86.3 5.7 10c 4 24.4 75.877.7 2072 25.0 16.5 84.6 8.2 10d 4 31.3 78.5 79.2 2194 26.9 15.0 81.911.9 10f 4 38.5 81.4 79.9 2298 28.9 11.9 79.4 15.1 11a 5 10.0 56.3 64.71714 13.8 83.3 85.2 6.9 11b 5 17.1 57.7 67.7 1813 14.4 32.1 79.2 15.011c 5 24.4 62.8 70.8 1899 14.6 23.3 70.4 24.3 11d 5 31.3 65.8 72.9 198720.2 20.2 64.4 28.9 11e 5 38.5 72.6 74.9 2119 24.5 17.7 57.3 33.4 12a 610.0 64.3 75.2 1732 22.0 25.5 86.2 5.8 12b 6 17.1 67.5 77.0 1798 24.416.2 84.9 7.8 12c 6 24.4 70.7 78.6 1897 25.7 13.6 81.7 11.5 12d 6 31.373.7 79.6 2024 28.1 11.0 78.9 14.5 12e 6 38.5 77.7 80.5 2178 29.3 9.975.6 18.1

Examples 13 to 15

4790 mL of dry cyclohexane were heated to 50° C. under inert conditionsin a 10 L stirred tank. sec-Butyllithium (s.-BuLi 1) in the form of a1.5 molar n-hexane solution was added, as was 0.4% by volume oftetrahydrofuran, based on the initial charge of cyclohexane, and themixture was stirred for 5 minutes.

After a first addition of styrene, the mixture was polymerized for 15minutes. A second addition of initiator (s.-BuLi 2) was made to initiatenew polymer chains, and further styrene was added in portions tocontinue block build-up. The reaction time for build-up of a styreneblock was 15 minutes, and that for a butadiene block was 40 minutes. Thepolymerization was terminated by adding 3 mL of isopropanol. 0.1% byweight of Kerobit TBK (2,6-di-tert-butyl-p-cresol), based on the solidscontent of the block copolymer, was added for stabilization. Table 4gives the amounts of styrene, butadiene, and sec-butyllithium used ineach of the stages, and also gives the properties of the resultant blockcopolymer mixtures. TABLE 4 Ex. 13 Ex. 14 Ex. 15 Stage 1 Styrene [g] 485716 716 s.-BuLi 1 [mL] 7.97 7.97 7.97 Stage 2 Styrene [g] 264 272 272s.-BuLi 2 [mL] 7.97 7.97 7.97 Stage 3 Butadiene [g] 411 411 351 Stage 4Styrene [g] 208 264 264 Butadiene content [%] 30 27 25 Proportion of 4441 41 1,2-linkages in polybutadiene [%] Mn [g/mol] 75600 85700 80800 PDI1.18 1.25 1.24 Tg [° C.] −65/98 −68/99 −67/99

Examples 16 to 18

The block copolymer mixtures of examples 13 to 15, in the form of 5%strength by weight solutions in cyclohexane, were hydrogenated by meansof a Pt/C hydrogenation catalyst (5% of Pt on activated charcoal) with apolymer/catalyst ratio by weight of 10:3, at 200° C. and 250 bar ofhydrogen, for 24 hours. Conversion of the GPC curves (TI signal) priorto and after hydrogenation showed no reduction in molecular weight. Theproperties of the hydrogenated block copolymer mixtures are given intable 5. TABLE 5 Ex. 16 Ex. 17 Ex. 18 Degree of hydrogenation 99.6 100100 [%] Modulus of elasticity 1.41 1.6 1.56 [GPa] Tensile strength [MPa]35.6 43.4 42.7 Tensile strain at break 85 70 62 [%] Tg [° C.] −55/127−54/133 −55/130 Vicat B [° C.] 113 113 115 Charpy, notched 23° C. 2.523.22 2.58 Transmittance [%] >92 >91 >91

Examples 19 to 21

In each case, a total of 20 g of ring-hydrogenated block copolymermixtures of examples 16 to 18, with ring-hydrogenated polystyrene (PCHEvia hydrogenation of PS 158 K from BASF Aktiengesellschaft), weredissolved in 200 mL of cyclohexane and stirred at room temperature. Thesolvent is then completely removed in vacuo at 80° C. The proportions byweight of PCHE and the properties of the blends are given in table 6.Modulus of elasticity, tensile strength, and tensile strain at breakwere determined on tensil specimens to ISO 3167 (all dimensions being ⅛of those of the master specimen), using a method based on ISO 527. TABLE6 Example 19a 19b 19c 20a 20b 20c 21a 21b 21c Block 16 16 16 17 17 17 1818 18 copolymer mixture from example Proportion by 10 17 50 10 17 50 1017 50 weight of PCHE [%] Modulus of 1.42 1.52 2.07 1.49 1.72 2.22 1.771.96 2.33 elasticity [GPa] Tensile 40.4 43.8 61.4 43.2 53 66.7 53 5668.8 strength [MPa] Tensile strain 117 97 8 69 69 6 71 67 6 at break [%]Tg [° C.] 139 140 143 140 145 140 140 143Transmittance >88 >86 >82 >89 >86 >90 >88 >86 [%]

1. A mixture comprising linear block copolymers made from vinylaromaticmonomers and from dienes of the structure (I) S₁—B₁—S₂ and (II)S₃—B₂—S₄, where S₁ is a block made from vinylaromatic monomers withnumber-average molar mass Mn in the range from 40,000 to 100,000 g/mol,each of S₂, S₃ and S₄ is a block made from vinylaromatic monomers withnumber-average molar mass M_(n) in the range from 5,000 to 20,000 g/mol,each of B₁ and B₂ is a block made from dienes or a copolymer block madefrom dienes and from vinylaromatic monomers with number-average molarmass M_(n) in the range from 15,000 to 40,000 g/mol, and the ratio ofthe block copolymers (I)/(II) is in the range from 0.6 to
 2. 2. Amixture as claimed in claim 1, wherein the ratio of the block copolymers(I)/(II) is in the range from 0.7 to 1.5.
 3. A mixture as claimed inclaim 1, wherein the ratio of vinylaromatic monomer to diene in theblocks B₁ or B₂ is in the range from 0.3 to 0.7.
 4. A mixture as claimedin claim 1, wherein each of the blocks B₁ and B₂ is a copolymer blockmade from dienes and from vinylaromatic monomers with randomdistribution.
 5. A mixture as claimed in claim 1, wherein thenumber-average molar mass M_(n) of S₁ is in the range from 45,000 to70,000 g/mol, that of each of S₂, S₃, and S₄ is in the range from 8,000to 17,000 g/mol, and that of each of B₁ and B₂ is in the range from18,000 to 30,000 g/mol.
 6. A mixture as claimed in claim 1, wherein theblocks S₃ and S₄ have the same number-average molar masses M_(n).
 7. Amixture as claimed in claim 1, wherein the number-average molar massesM_(n) of S₂ and S₄ are identical, and the number-average molar masses ofB₁ and B₂ are identical.
 8. A mixture as claimed in claim 1, whichcomprises from 5 to 95 percent by weight of the linear block copolymers(I) and (II) and from 95 to 5 percent by weight of standard polystyreneor impact-modified polystyrene.
 9. A mixture, obtainable by partial orcomplete hydrogenation of the mixture as claimed in claim
 1. 10. Amixture as claimed in claim 9, wherein the degree of hydrogenation ofthe vinylaromatic monomer units is at least 90%.
 11. A mixture asclaimed in claim 9 or 10, wherein, prior to the hydrogenation, and basedon the entirety of the diene units, more than 30% of the copolymerizeddiene units are present in the 1,2-vinyl form.
 12. A process forproducing mixtures as claimed in claim 1, which comprises preparing eachof the block copolymers (I) and (II) by sequential anionicpolymerization of vinylaromatic monomers and of dienes withorgano-alkali metal initiators in succession or in different reactors,and then blending these with a ratio (I)/(II) in the range from 0.6 to2.
 13. A process for preparing mixtures as claimed in claim 7, whichcomprises preparing the block copolymers (I) and (II) by sequentialanionic polymerization of vinylaromatic monomers and of dienes usingorgano-alkali metal initiators simultaneously in one reactor using twojoint feeds of initiator and vinylaromatic monomers, where the ratio ofthe amount of initiator 11 in the first feed to the amount of initiator12 in the second feed is in the range from 0.6 to
 2. 14. A process forpreparing mixtures of ring-hydrogenated block copolymers encompassingthe steps of: a) sequential anionic polymerization of the vinylaromaticmonomers and dienes as claimed in claim 12, b) termination of thepolymerization, using a protic terminator or a coupling agent, c)hydrogenation of the resultant block copolymer using a hydrogenationcatalyst.
 15. A process as claimed in claim 14, wherein step a) iscarried out in a cycloaliphatic hydrocarbon as solvent, and in thepresence of from 0.3 to 0.5% by volume, based on the solvent, of anether.
 16. The use of the block copolymers as claimed in claim 9 foroptical media.